Add notpossible keyword

This is the same as `unreachable` except it always traps violations,
even if we're not running in MODE=dbg. This is useful for impossible
conditions relating to system calls. It avoids terrifying bugs where
control falls through to an unrelated function.

libc/sock/nointernet.c

@@ -66,8 +66,7 @@
   do {                                                        \
     if (UNLIKELY((x) == -1)) {                                \
       DEBUG("%s:%d: %s failed %m\n", __FILE__, __LINE__, #x); \
-      asm("hlt");                                             \
-      unreachable;                                            \
+      notpossible;                                            \
     }                                                         \
   } while (0)